grinding medium, to make them petrol-proof; as purchased, they nearly always leak. The small cocks required in

the petrol - pipe line will, unless specially made, also require regrinding; the best plan is to discard the use of

plug cocks altogether and use screw-down needle valves instead. As well as being proof against leakage, needle valves are less likely to become choked, being to a certain extent self-clearing. For making joints in the petrol pipes, red lead, rubber, and such-like are useless. Flexible vulcanized fibre can be relied on, and when something to take the place of red-lead cement is wanted, use soap. This, being insoluble in petrol, makes an excellent jointing material.

When making standard drawings for motors, it will be found convenient to keep cast and wrought work on separate sheets as far as possible, grouping the component parts with a view to the various shops concerned in carrying out the designs. The writer has also found it advisable to have two sizes of drawings-for general arrangements, 30 x 22 inches; and for details, 22 x 15 inches. The smaller sheets are more convenient for the workmen to handle, while the larger, not being in such constant use, do not get in the way.

Cooling.-Owing to the great heat developed within the cylinder of an internal combustion engine (see p. 19), it is necessary to employ extraneous means for keeping the cylinder cool enough to permit of proper lubrication. The cooling system is not intended, as is sometimes thought, to abstract the heat from the gases within the cylinder, but is provided to cool the cylinder walls only. The gases themselves should retain as much as possible of the heat due to combustion, hence it is advisable to let the cylinder work at as high a temperature as is consistent with efficient lubrication. It should also be the aim of the designer to arrange the cooling system so that the temperature of the cylinder may be kept as uniform as possible.

Air-cooling is limited to engines of small dimensions, though many attempts are being, and have been, made to

apply it to motors of forty horse-power, especially in America. Air-cooled engines are very liable to become overheated, when the piston is apt to bind in the cylinder. Also the incoming charge of air and gas is likely to be

G

Taper of Cone

FIG. 25.

considerably attenuated by the expansion due to the mixture being heated as it flows into the combustion chamber. By the judicious use of fans, giving a rapid circulation of air around the combustion chamber, the effectiveness of air-cooling may be somewhat increased, but the fact that a fan takes a certain amount of power to